heat exchangers in existing ac systems.

I would like to add a diesel fired hydronic furnace to the boat... The boat has 3 reverse cycle ac systems each with vents to specific areas of the boat. has anyone just added heat exchangers from the hydronic furnance to the existing venting.... the heat exchangers I'm looking at have their own fan. It seems using the existing venting would simplify the installation. thoughts?

Ka_sea_ta said:

I would like to add a diesel fired hydronic furnace to the boat... The boat has 3 reverse cycle ac systems each with vents to specific areas of the boat. has anyone just added heat exchangers from the hydronic furnance to the existing venting.... the heat exchangers I'm looking at have their own fan. It seems using the existing venting would simplify the installation. thoughts?

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I'm guessing that you're considering using a hot water coil? If it's hydronic, it's a boiler. If it's a furnace, it's hot air. Different systems.

To use just the existing duct system would require the implementation of a damper that would essentially be a "wye valve", that would connect the duct system to whichever piece of equipment is in operation. If you have the room, it could be possible to do. It would require an interlock to prevent operation of the system that is not connected to the duct system. There would also need to be some provision to change between the systems. Automating that changeover could be challenging. It's not something that's commonly implemented, so a COTS solution would be unlikely.

If they have their own fans, not sure how you can add them to existing ducting witithout obstructing them for A/C use or if you T into them, the flow would just split and go hslf back toward the A/C unit....or at least not be efficient.

To expand a bit on Steve's advice above and disagree a little bit, let me add my two cents:


There isn't really anything bad that will happen if you run the heat with the AC duct open- the air will just come out the AC return air grill as well. There really isn't anything terribly bad if you happen to run both AC and heat at the same time. You will waste a lot of energy and maybe freeze up the AC coil due to low air flow since there will be two fans running in the system simultaneously and that will result in low air flow from both.


So, if you don't want all of the interlocks and automatic dampers that Steve describes, you could just plumb the heater fan output into the AC ductwork and put a manual damper in front of both the heater fan output and the AC fan output and just remember to set them right manually.


It still takes a lot of room in tight places to do this and most boats don't have that room.


David

I very briefly considered what you're describing when refitting my boat. Also considered the heat coil with its own 4" duct and a very shallow "Y" to inject the heat into the duct without any air valves. Each consideration was rejected as I felt there would be AC performance loss. Ended up installing 3 surface mount buss style heaters instead. Mine utilize engine heat when underway. Very happy with the results. Was able to maintain mid 70s inside boat temperature on a cloudy day while crossing mid 30s water temperature on Lake Superior.

Ted

psneeld said:

If they have their own fans, not sure how you can add them to existing ducting witithout obstructing them for A/C use or if you T into them, the flow would just split and go hslf back toward the A/C unit....or at least not be efficient.

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My initial thought was to use a "Y" in the duct line. I'll talk to the guys at Sure and see if this is even feasible. I'm a HVAC novice at best but thought if I could use the existing venting why not

I thought you meant putting heat exchangers inline with the existing duct work, and running the AC on fan only for heat. It would work but requires some custom parts and engineering. Most of the small heat exchangers with fans that I've seen do not have the the CFM or pressure to push much heat through AC ducts. AC units have a large radial fan with lots of CFM even with some back pressure. The hydronic heat exchangers have a small axial fan and work best with small, short ducts or no ducts at all.

I think a simple way to do what you suggest is to just put a heater core coil like a car has in a small plenum behind each return air grille. Run hot water thru the coils and put the ac on fan only. The air drawn in the return will be heated by the coil in the plenum and distributed to the outlets just as it is now. Controlling temp could be done by a common thermostat cycling the boiler or water circulation pump on and off. Multizone I guess could be controlled by individual thermostats and zone valves controlling the water flow. If you could get it right it would be nice heat. Best if the fans had a low speed setting (less noise).

To prevent undesired backflow of air, you might be able to install automatic dampers, one in each of the AC and heat outlets. Then whichever is operating would push its flap open and the other flap closed. That would be a very simple form of automatic flow control.

Another consideration is where the ac ducts blow into the living space. AC ducts usually exit high up in the room since cold air drops, and gives more even cooling. In an ideal world, you want heat to enter the room down low since it rises, and will give similarly even heating. Heating via the AC ducts won’t give as even heating. Whether that’s worth the work to duct them separately? You’ll have to decide.